A partial oxidation-based approach to the synthesis of gold-magnetite hybrid nanostructures

Hybrid nanostructures composed of gold and magnetite are of singular interest because they allow the integration of plasmonic and magnetic properties in a single object. Due to this feature, their application has been proposed to perform various functions. The methods usually employed to prepare these particular kinds of nanostructures follow organic phase routes, whereas synthetic methodologies that employ more sustainable solvents have been much less explored. In this work, an environmentally friendly approach for the synthesis of gold-magnetite hybrid nanostructures in aqueous media is proposed. This approach relies on the partial oxidation of the Fe(II) precursor using hydrogen peroxide as the oxidizing agent in the presence of preformed gold nanoparticles dispersed in the reaction medium. The methodology used led to the formation of magnetite nanoparticles with a good stoichiometry and a median size of 30 nm. Furthermore, in the presence of gold nanoparticles in the reaction medium, the formation of gold-magnetite hybrid nanostructures is produced as a consequence of the heterogeneous nucleation of the iron oxide phase on the surface of the gold nanoparticles that act as seeds. The approach reported broadens the possibility of synthesizing hybrid nanostructures in aqueous media with integrated plasmonic and magnetic properties.

This document provides more detailed information to the main paper mentioned above.The following information is included:

Estimation of the concentration of Au NPs
The estimation of the Au NPs concentration was performed based on the Lambert and Beer law and on the assumption that the synthesized Au NPs consists of Au nanospheres of a single diameter.
Considering that, the extinction cross section of a Au nanosphere, Cext, can be expressed as: where NA is the Avogadro's number, and D and Qext stand by the diameter and extinction efficiency of the nanosphere, respectively, the measured absorbance A is given by: where b is the optical path length of the cuvette (1 cm), and C is the concentration of Au NPs.The fitting of the measured extinction spectrum to spectra simulated with the Mie theory allows to determine D and Qext and therefore Cext.The extinction spectrum of the synthesized Au NPs is shown in SI Figure 2, black curve.On the other hand, the red curve represents the simulated extinction efficiency spectrum of a D=57 nm Au nanosphere dispersed in water, which has a Qext = 4.74 at λ = λLSPR, that is λ = 535 nm, resulting in Cext = 2.9 x 10 14 cm 2 /mol.Taking into account that A = 0.269 at λ = 535 nm, it turns that C = 9.2 x 10 -16 mol/cm 3 or 9.2 x 10 -13 mol/L, a value that can be expressed approximately as ~ 1 x 10 -12 M.  Visual change in the color of the colloidal solution after the formation of Au-Fe3O4 HNs The pink color of the Au NPs suspensions changes to a brown color after formation of the Au-Fe3O4 HNs.The very good agreement between both spectra indicates that a tolerable error is reached when using an inter-dipole distance of 1 nm to model the Fe3O4 nanosphere.In addition, the experimental extinction spectrum of the synthesized Fe3O4 NPs (see Figures 2a and 2b) is shown in red curve.The profiles of the simulated spectra and of the experimental spectrum measured for the Fe3O4 NPs aqueous dispersion are quite similar.The larger experimental extinction values with respect to the simulated ones are attributed to larger scattering contributions from the bigger Fe3O4 NPs of the distribution (see Figure 2b) as well as to aggregates of Fe3O4 NPs that might be dispersed in solution.

Figure S2 .
Figure S2.Extinction spectrum of the purified Au NPs (black curve) and simulated extinction

Figure S3 .
Figure S3.(a) Representative TEM image and (b) number density distribution of the size, q0, of the

Figure S5 .
Figure S5.Comparison between the normalized extinction efficiency spectra of a 57 nm diameter

Figure S6 .
Figure S6.Comparison between the normalized extinction efficiency spectra of a 30 nm diameter